Browsing by Author "Caldwell, Patricia May."

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Chemical control of soybean rust (Phakopsora pachyrhizi) on soybeans.
(2005) Du Preez, Eve Diane.; Caldwell, Patricia May.; Laing, Mark Delmege.
Soybean rust (SBR) caused by Phakopsora pachyrhizi Syd. is an aggressive wind dispersed fungal disease which has spread around the world at an alarming rate in the last decade. The disease was first reported in South Africa (SA) in 2001. It has become well established in the province of KwaZulu-Natal. Reports are occasionally made from eastern Mpumalanga, late in the growing season, in years with good rainfall. Yield losses of 10 - 80% have been reported due to SBR infection. Literature was reviewed to better understand the pathogen in an attempt to find suitable disease management strategies. Many strategies involve delaying, rather than preventing, SBR infection. Of the two strategies to prevent infection, the use of fungicides was the only option for disease control in SA, as no resistant cultivars are available. Field trials were conducted to determine which fungicides are effective in controlling SBR. Further research was conducted to determine the timing, frequency and rate of fungicide applications for optimal control of SBR. Trials were evaluated for disease severity, seed yield and the effect of fungicides on seed quality. Fungicides from the triazole class of the sterol biosynthesis inhibiting group of fungicides were found to be the most effective in controlling SBR. A fungicide from the strobilurin group was found to be less effective than the triazoles at the suggested rate, but was found to be as effective when evaluated at higher dosage rates. Triazoles premixed with fungicides from the benzimidazole and strobilurin groups were also effective in controlling SBR. Timing of application was found to be critical for strobilurin fungicides, but not for triazole fungicides, which have a curative ability, unlike strobilurins. Strobilurin fungicides applied preventatively, before the appearance of disease symptoms were as effective as triazole fungicides applied after disease symptoms, but before infection levels had reached 10%. Across both wet and dry seasons two fungicide applications applied at 21d intervals at the R2 growth stage resulted in effective disease control. In wet seasons, a third fungicide application resulted in yields that were higher, albeit not statistically significant, than two fungicide applications. Assessments of individual fungicides for optimal dosage rate found that registered rates were already optimal for some fungicides, but for others it appeared as if alterations were necessary to the rate suggested for registration. This study was one of the first to extensively evaluate the efficacy of the new triazole and strobilurin fungicides on SBR control. The results have been shared globally, but particularly with newly affected countries in South and North America. Although this research has been groundbreaking, there are still many aspects of fungicide control which need to be studied in order to further optimise chemical control of SBR.
The effects of Trichoderma (Eco-T) on biotic and abiotic interactions in hydroponic systems.
(2003) Neumann, Brendon John.; Laing, Mark Delmege.; Caldwell, Patricia May.
The following body of research provides a detailed overview of the interactive effects of biocontrol agents and environmental factors and how these influence both the host plant and pathogen populations within hydroponic systems. Pythium and other zoosporic fungi are pathogens well suited to the aquatic environment of hydroponics. Motile zoospores facilitate rapid dispersal through fertigation water, resulting in Pythium becoming a yield reducing factor in most hydroponic systems and on most crops. With increasing trends away from pesticide use, biocontrol is becoming an ever more popular option. Unfortunately, much of our knowledge of biocontrol agents and their formulation can not be directly transferred to the widely differing environments of hydroponic systems. Paulitz (1997) was of the opinion that if biocontrol was to be successful anywhere, it would be in hydroponics. This is primarily due to the increased ability, in hydroponics, to control the growing environment and to differentiate between the requirements of the pathogen versus those of the host plant and biocontrol agent. Key environmental factors were identified as soil moisture, root zone temperature, form of nitrogen and pH. A review of the literature collated background information on the effects of biocontrol agents and environmental manipulation on plant growth and disease severity in hydroponic systems. A commercial formulation of Trichoderma (Eco-T(R1)) was used as the biocontrol agent in all trials. Dose responses in Pythium control and plant growth stimulation in lettuce were first determined using a horizontal trough system (closed system). In such systems optimum application rates were found to be lower than in field application (1.25x10[to the power of 5] spores/ml). This is probably because Trichoderma conidia are not lost from the system, but re-circulate until being transported into the root zone of a host plant. No significant growth stimulation was observed, although at high doses (5x10[to the power of 5] and 2.5x10[to the power of 5] spores/ml) a significant reduction in yield was recorded. Possible reasons for this growth inhibition are suggested and a new theory is proposed and investigated later in the thesis. In an open system of cucumber production (drip irrigated bag culture) no statistically significant results were initially obtained, however, general trends still showed the occurrence of positive biocontrol activity. The initial lack of significant results was mostly due to a poor knowledge of the horticulture of the crop and a lack of understanding of the epidemiology behind Trichoderma biocontrol activity. These pitfalls are highlighted and, in a repeat trial, were overcome. As a result it could be concluded that application rates in such systems are similar to those used in field applications. Management of soil moisture within artificial growing media can aid in the control of Pythium induced reductions in yield. A vertical hydroponic system was used to determine the interactive effects of soil moisture and Trichoderma. This system was used because it allowed for separate irrigation regimes at all 36 stations, controlled by a programmable logic controller (PLC). With lettuce plants receiving optimum irrigation levels, no significant reduction in yield was observed when inoculated with Pythium. However, after Pythium inoculation, stresses related to over- or under-watering caused significant yield losses. In both cases, Trichoderma overcame these negative effects and achieved significant levels of disease control, especially under higher soil moisture levels. Growth stimulation responses were also seen to increase with increasing soil moisture. Similar results were obtained from strawberry trials. These results show that Pythium control is best achieved through the integration of Trichoderma at optimum soil moisture. However, where soil moisture is above or below optimum, Trichoderma serves to minimize the negative effects of Pythium, providing a buffering capacity against the effects of poor soil moisture management. Pythium, root zone temperature and form of nitrogen interact significantly. In greenhouse trials using horizontal mini troughs with facilities for heating or cooling recirculating water, nitrate fertilizer treatments resulted in statistically significant results. Lettuce growth was highest at 12°C, although no significant differences in yield were observed between 12-24°C. Pythium was effective in causing disease over the same temperature range. Pythium inoculation did not result in yield reduction at 6 and 30°C. Trichoderma showed a slight competitive advantage under cooler temperatures (i.e., 12 degrees C), although significant biocontrol occurred over the 12-24 degrees C range. Ammonium fertilizer trials did not generate statistically significant data. This is possibly due to complex interactions between root temperature, ammonium uptake, and competitive exclusion of nitrification bacteria by Trichoderma. These interactions are difficult to replicate over time and are probably influenced by air temperature and available light which are difficult to keep constant over time in the system used. However, the data did lead to the first clues regarding the effects of Trichoderma on nitrogen cycling as plants grown with a high level of ammonium at high temperatures were seen to suffer more from ammonium toxicity when high levels of Trichoderma were added. In further trials, conducted in the recirculating horizontal mini trough system, it was determined that Trichoderma applications resulted in an increase in the percentage ammonium nitrogen in both the re-circulating solution and the growing medium. This was a dose-related response, with the percentage ammonium nitrogen increasing with increasing levels of Trichoderma application. At the same time an increase in ammonium in the root tissue was observed, corresponding with a decrease in leaf nitrate levels and an increase in levels of Cu, Na, Fe and P in leaf tissue. In independent pot trials, populations of nitrifying bacteria in the rhizosphere were also seen to decrease with increasing Trichoderma application rates. This led to the conclusion that the increase in ammonium concentration was as a result of decreased nitrification activity due to the competitive exclusion of nitrifying bacteria by Trichoderma. The possibility that Trichoderma functions as a mycorrhizal fungus and so increases the availability of ammonium for plant uptake is not discarded and it is thought that both mechanisms probably contribute. Water pH provides the most powerful tool for enhancing biocontrol of Pythium by Trichoderma. Trichoderma shows a preference for more acidic pHs while Pythium prefers pHs between 6.0 and 7.0. In vitro tests showed that Trichoderma achieved greater control of Pythium at pH 5.0, while achieving no control at pH 8.0. In greenhouse trials with the recirculating horizontal mini trough system, yield losses resulting from Pythium inoculation were greatest at pH 6.0 and 7.0, with no significant reduction in yield at pH 4.0. Biocontrol activity showed an inverse response with greatest biocontrol at pH 5.0.
Evaluation of the potential use of antagonistic microbes on grass species, turf and pasture, for disease control and growth stimulation.
(2003) Cunningham, Debra M.; Laing, Mark Delmege.; Caldwell, Patricia May.
Public tendency, of late, is to reduce liberal use of harmful synthesized chemicals for promoting plant health. Today, biological control is becoming a commonly cited disease control option. Biological control agents (BCAs) not only control disease , but also promote plant growth. Application of biological control is based largely on knowledge of control mechanisms employed by antagonists, as well as the means of application that will ensure that an antagonistic population is established. Knowing the advantages is not the only factor that should be considered before application commences as, the disadvantages must be clearly outlined and explored further before a constructive decision as on implementation of biological control. A literature review was undertaken to provide the necessary technical information about biological control, its potential uses, methods of application, mechanisms of action employed, advantages and disadvantages associated with biological control application, public perceptions and the potential future of biological control. Diseases encountered within the KwaZulu-Natal Midlands on pasture and turf grasses were determined by a once-off survey conducted over 1999/2000. The aim of the survey was to determine broadly the management practices of farmers and groundsmen in KwaZulu-Natal and the potential impact of these on the occurrence of weeds, insects and diseases. The survey also addressed the level of existing knowledge about biological control and willingness to apply such measures. In the pasture survey, farmers were questioned about: soil type, grass species common used, irrigation , fertilization and liming, grazing programs and weed, insect and disease occurrences and control measures implemented. The same aspects were addressed in a survey to a representative sample of groundsmen (turfgrass production) , including also: topdressing, greens base used, drainage systems, mowing practices and decompaction principles. The survey showed correlation between pest incidence and management practices implemented. In terms of pest control, both farmers and groundsmen indicated a stronger preference to the use of herbicides , insecticides and fungicides. Use of fungicides for disease control by farmers is considered an often unfeasible expense, rather more emphasis was placed on implementing cultural control methods. At present farmers do not apply biological control strategies, but they did indicate much interest in the topic. Alternatives to current, or lack of current, disease management strategies are important considerations, with two new diseases identified in the KwaZulu-Natal Midlands just within the period of this thesis. Biological control strategies are implemented by 8% of the groundsmen surveyed, with emphasis being placed on augmenting the already present natural predators rather than the introduction of microbial antagonists. Although often mis-diagnosed by farmers Helminthosporium leaf spot is a common disease in the KwaZulu-Natal Midlands on Pennisetum clandestinum (kikuyu), This disease reduces pasture quality and detracts from the aesthetic appearance and wearability of turfgrasses. Helminthosporium leaf spot is incited by a complex of causal agents , Bipolaris was confirmed as the casual agent of Helminthosporium leaf spot on kikuyu at Cedara. Disease control by two BCAs, Bacillus (B. subtilis Ehrenberg & Cohn.) and Trichoderma (T. harzianum Rifai), as commercial formulations was tested against the fungicide, PUNCH EXTRA®. In vitro, Trichoderma was shown to be aggressive in controlling Bipolaris sp. In vivo, disease control achieved with Trichoderma kd was comparative with PUNCH XTRA® but not statistically different (P>=0.05). Trichoderma and Bacillus provided better disease control in comparison to an untreated control. Improved growth of Lolium sp. was determined in vitro, with Trichoderma kd and Bacillus B69 treatments. The microbe-based treatments accounted for growth stimulation, with significant (P<=O.05) growth differences noted. A microbial activator, MICROBOOST®was added to the treatments to improve microbial efficiency. Improved plant growth with MICROBOOST® applications was shown. Improved growth associated with microbial treatments, Trichoderma harzianum kd; Bacillus subtilis B69 and Gliocladium virens Miller, Gibens, Foster and con Arx. ,was also determined in vivo at Cedara, on L.perenne L., Festuca rubra L. and Agrostis stolonifera L. Establishment of a suppressive soil with antagonistic microbes resulted in significant (P<=O.05) effects on final grass coverage (except G. virens), as well increased root and shoot lengths (P<=O.05). Increased germination rates, as expressed in vitro, were not shown in vivo. Microbial activity with the application of MICROBOOST® showed little effect on germination but increased root and shoot lengths significantly (P<=O.05). Increased weed growth associated with the treatments (except G. virens) was considered a drawback of the microbial-treatments. Microbial treatments were also applied to pasture grasses. An in vitro grazing trial was established at Cedara, using L. multiflorum L. to evaluate the microbe-based treatments Trichoderma kd, Bacillus B69 and G. virens for improved pasture establishment and for increased grazing preference by Dohne Merino sheep. Trichoderma kd was associated with increased dry and wet biomass , but lower dry matter yields in comparison to the control. Only G. virens accounted for a higher dry matter percentage than the control. However, differences between the control and the microbial treatments was very small and not significant (P>=0.05). Of the three grazing observations made, sheep showed no grazing preference to plots with or without microbial treatments In general, the body of this research has shown that microbial treatments have the potential for increased disease control and growth stimulation of grasses. However, lack of significant differences between microbial treatments and controls has raised the question as to effect of external factors on microbial activity and survival, especially in vivo. This raises the question as to the validity of the use of microbial treatments where growth conditions cannot be controlled , remembering that the cost of establishment must be covered by the economic returns from utilization.
Investigating induced resistance in sugarcane.
(2013) Edmonds, Gareth John.; Laing, Mark Delmege.; Caldwell, Patricia May.
Five potential resistance-inducing chemicals were applied to two sugarcane varieties (N12 and N27) in a pot trial with the aim of inducing resistance to nematodes in naturally-infested soil. BION® (acibenzolar-S-methyl), methyl jasmonate, cis-jasmone and 2,6-dichloroisonicotinic acid (INA) were applied as a foliar spray and suSCon® maxi (imidacloprid) applied to the soil. All chemicals were tested at two rates and plants were sprayed one week prior to being harvested at 7, 9 and 11 weeks of age. Meloidogyne and Pratylenchus infestation of sett and shoot roots was determined at each harvest. The activity of four pathogenesis-related proteins was examined at 7, 9 and 11 weeks using separate assays, these enzymes where chitinase, β-1,3-glucanase, peroxidase and polyphenol oxidase. Methyl jasmonate treatment produced significant increases in β-1,3-glucanase, chitinase and peroxidase activity. All other elicitor treatments showed little difference in enzyme activity from the Control. The effect of each treatment on plant growth was examined by recording the dried root and shoot biomass of each plant. No significant differences were seen (p<0.05; Holm-Sidak test). However, root and shoot dried biomass was highest in the N12 variety treated by suSCon® maxi. The infection of sugarcane with Ustilago scitaminea (sugarcane smut) is commonly identified visually by the presence of a smut whip. Identification of sugarcane smut infection can be determined prior to whip development by staining tissue sections with lactophenol cotton blue and examining plant tissues microscopically. This allows for a rapid determination of smut infection which can aid breeding programs. Smut infection is achieved in vitro by soaking sugarcane setts in smut spores collected from infected whips. Four methods of inoculation were examined. The method that most consistently caused infection involved allowing setts to germinate for 24 hours, before puncturing a bud with a toothpick, followed by submerging the sett in 1x10⁸ smut spores per mℓ. An elicitor of systemic acquired resistance called BION®, and an insecticide with resistance-inducing properties called Gaucho® (imidacloprid) were used as a sett soak treatments to induce resistance to sugarcane smut. The effect of each treatment at three concentrations on plant germination and growth was examined in the NCo376 variety. Smut spore germination on agar was examined in the presence of both treatments at three concentrations. Sugarcane setts were treated with a concentration that did not significantly reduce the germination of smut spores or sugarcane setts. Plants were infected with smut post treatment and allowed to grow for approximately one month until plants were between 8 and 10 cm in height. Smut infection was assessed by cutting longitudinal sections through the base of the shoot and staining each section with cotton blue lactophenol. Treatment with BION® and Gaucho® did not reduce smut infection.
Studies of integrated control of selected root diseases of sunflowers using Trichoderma harzianum (ECO-T®) and silicon
(2009) Elungi, Konis.; Caldwell, Patricia May.
The soil-borne fungi Rhizoctonia solani Kuhn and Sclerotinia sclerotiorum De Bary are ubiquitous plant pathogens with a wide host range. They are among the most widespread and destructive diseases of many crops, including sunflowers. Although in many cases, the use of chemicals appears to be the most economical and efficient means of controlling plant pathogens, their environmental concerns and the development of tolerance in pathogen populations have led to drastic reduction in their usage and increased the need to find alternative means of disease control. The potential benefits of applying Trichoderma harzianum Rifai and silicon (Si) nutrition to plants have been extensively reviewed. In this study, the ability of T. harzianum (Eco-T®), soluble silicon, and their combination was evaluated on sunflower (Helianthus annuus L.), for their potential to suppress pathogenic strains of R. solani and S. sclerotiorum. The ability of this crop to take up and accumulate Si in different plant parts was also investigated. In vitro assessment of fungal responses to Si in PDA showed that both R. solani and S. sclerotiorum were inhibited in the presence of Si. More inhibition was observed as the Si concentration increased with a relative increase in pH. Maximum growth inhibition was observed at 3000 mg ;-1 – 6000 mg ;-1 of PDA. No difference in inhibition between the two pathogens was observed, thus confirming the fungitoxic/suppressive ability of high Si concentrations to fungal growth. In addition, in vivo trials showed that the Si concentration of 200 mg ;-1 applied weekly significantly increased the dry weight of plants inoculated with R. solani and S. sclerotiorum and was therefore considered the optimum concentration. Assessments on in vitro antifungal activities of Eco-T® on R. solani and S. sclerotiorum, showed that Eco-T® significantly inhibited mycelial growth, in both dual culture methods and volatile and non-volatile compounds produced by Eco-T®. In addition, the combination of Eco-T® and Si was most effective in suppressing damping-off and increasing plant dry weight of sunflower seedlings in the greenhouse. The combination of Si and Eco-T® significantly increased percentage germination, number of leaves and head dry weight of the sunflower cultivars tested. Silicon alone increased growth but was unable to control R. solani and S. sclerotiorum effectively. Rhizotron studies showed that S. sclerotiorum infected the host through the roots and the stem, whereas R. solani only infected the host through the roots. A study on Si uptake and distribution showed that sunflower accumulates Si in various plant tissues. Analysis of plant tissues revealed that more Si was accumulated in leaves > stems > roots, with the Si levels in leaves being significantly higher than in stems and roots. In conclusion, Si alone could be used to increase growth but was unable to control R. solani and S. sclerotiorum. However, Si together with Eco-T® provides an environmentally friendly alternative for the control of R. solani and S. sclerotiorum, and enhanced plant growth and yield.
Studies on Cercospora zeae-maydis, the cause of grey leaf spot of maize in KwaZulu-Natal.
(2000) Caldwell, Patricia May.; Laing, Mark Delmege.; Wallis, Frederick Michael.; Rijkenberg, Fredericus Hermanus Johannes.
In 1983, Latterell and Rossi described grey leaf spot (GLS) of maize (Cercospora zeae-maydis Tehon and Daniels) as "a disease on the move". This pathogen has more than lived up to its reputation. It is estimated to be spreading at a rate of 80-160 km each year, and is recognized as one of the most grain yield-limiting diseases of maize worldwide. The occurrence of the pathogen in the Province of KwaZulu-Natal (KZN), Republic of South Africa (RSA), in 1988, was its first official report from the African Continent. It has since become pandemic, causing grain yield losses of up to 60%. It has spread to other provinces in RSA as well as other African countries, namely Cameroon, Kenya, Malawi, Mozambique, Nigeria, Swaziland, Tanzania, Uganda, Zaire, Zambia and Zimbabwe. It has also been reported to occur in Brazil, China, Columbia, Costa Rica, Mexico, Peru, Trinidad, and Venezuela. The use of soil macro- and micronutrients in the management of fungal plant pathogens is widely documented in the literature. Specific nutrients are known to increase or decrease disease resistance in plants. However, each host-pathogen interaction must be considered on an individual disease basis, together with environmental and soil variables. Although few diseases can be eliminated by a corrective fertilizer regime, the severity of a disease can be reduced by specific nutrients, particularly when used in conjunction with other cultural practices. However, the economic implications, and not grain yield alone, of different control measures should be considered; i.e., farmers must compare the expected added gross margin ha -1 (added income minus added costs) with the potential variability in expected added gross margin ha -1 (upper and lower limits) of each treatment when deciding on which fertilizer applications and/or fungicide treatments to use. Literature reviews were undertaken on both GLS and the use of soil nutrients to control fungal plant pathogens to provide the necessary background technical information in order to conduct research under local conditions, and to assist in interpretation of results of experiments. Nutrient trials to control GLS were conducted at two sites in KZN, i.e., Cedara (1995/96, 1996/97 and 1997/98) and Ahrens (1995/96). Research at Cedara showed that with increased applications of nitrogen (N) at 0, 60 and 120 kg N ha -1 and potassium (K) at 0, 25, 50 and 150 kg K ha -1, leaf blighting occurred earlier, and final percentage leaf blighting and the standardized area under disease progress curve were higher. The Ahrens trial also showed that with increased applications of N (0, 60, 120 and 180 kg N ha -1) and K (0, 50, 100 and 150 kg K ha -1), there were also increases in final percentage leaf blighting. Increasing phosphorus levels of 0, 30, 60 and 120 kg P ha -1 did not have any effect on final percentage leaf blighting. The application of systemic fungicides to GLS-susceptible maize was highly effective in controlling GLS and increasing grain yields substantially with increased N and K applications. In the non-fungicide treated plots, grain yields did not increase with increased applications of K in all three years of the trial. This was probably because grain yield response, which should have occurred at higher K applications, was reduced by increased GLS severity. Similarly, grain yields did not increase significantly with N application in 2 of the 3 years of the trial. At Cedara, non-fungicide treated maize produced a financial loss of -R165 and -R48 with 25 and 50 kg K ha -1 respectively, relative to 0 kg K ha -1. However, increasing N applications resulted in increasing grain yields, and added gross margins of R714 ha -1 and R536 ha -1 with applications of 60 and 120 kg N ha -1, respectively. The drop in added gross margin at 120 kg N ha -1 was probably because of increased GLS levels at higher fertiliser rates, resulting in reduced grain yields. In fungicide treated maize, added gross margin relative to 0 kg K ha -1 increased from R851 to R1212 ha -1. However, there was a loss of -R133 ha -1 in added gross margin relative to 0 kg N ha -1 at 60 kg N ha -1 as increased grain yields did not offset the added cost of N fertilizer and fungicide applications. At 120 kg N ha -1 added gross margin relative to NO was R423 ha -1. Highest grain yields and gross margins in fungicide treated maize were obtained with 120 kg N ha -1 and 150 kg K ha -1, as expected. However, in non-fungicide treated maize, highest grain yields and gross margins were obtained using 60 kg N ha -1 and 50 kg K ha -1. This was because of higher GLS severity at the higher N and K application rates. Yields of wheat grown in soils with residual fertilizers after non-fungicide treated maize were higher (4.21 ha -1) compared to yields (3.61 ha -1) grown on residual fertilizers after maize that had been sprayed to control GLS. This was probably as a result of GLS reducing the photosynthetic area of maize leaves, causing premature death with a concomitant reduced uptake of nutrients by roots. This resulted in higher residual levels of fertilizers in soils where fungicide applications were not used to control GLS on maize compared to soils planted with maize where GLS was controlled through the application of fungicides. In KZN there are approximately 350,000 small-scale farmers. The same diseases that affect commercial agricultural production also affect the small-scale farmer, the major difference being in the methods of disease control employed. At the commercial level, most farmers rely on the use of agro-chemicals, which are often not available to the small-scale farmer due to the relatively high cost of agro-chemicals, application methods, and the non-availability of products in the rural areas. The level of illiteracy of the small-scale farmer may also inhibit the use of agro-chemicals. In many African countries, the per capita consumption of maize may be as high as 100 kg per year. Production of cereals in Africa has fallen in the past 25 years. This, together with yield reductions of maize caused by GLS, is likely to contribute to an even greater food deficit in many African countries. At present, low soil fertility and pH levels are a problem among small-scale farmers both in the RSA and other parts of Africa. In the RSA, government policy is to increase maize production by small-scale farmers through improved agronomic methods, including increased fertilizer application. Appropriate and affordable rotations and other improved agronomic practices need to be developed and promoted to ensure food security and sustainable systems for smallscale farmers. The results from the nutrient trials presented in this thesis have practical applications for the small-scale farmer who does not have the option of controlling GLS through the use of agrochemicals. The small-scale farmer will be able to attain a maximum gross margin from his maize crop by applying 60 kg N ha -1 and 50 kg K ha -1, if no fungicides are applied. However, comparative analyses of manure showed that a small-scale farmer would have to apply 1-3 tonnes of manure in order to achieve similar nutrient levels - a procedure that would be impractical. Comparative financial analyses of aerial and knapsack fungicide applications showed that it would be uneconomical for the small-scale farmer to apply fungicides using a knapsack sprayer. A simple spreadsheet has been created to help farmers make the best choice of N (0, 60 or 120 kg N ha -1) and K (0, 25, 50 or 150 kg K ha -1) and the number of fungicide application (O, 1, 2 or 3). This will eliminate the guesswork needed for farmers to maximize gross margins, based on a specific amount of money available. The resistance expressed by different hybrids on conidial germination of C. zeae-maydis at varying temperatures, desiccation periods and interrupted dew periods was investigated using the susceptible ZS 206 and the less susceptible SC 625 maize cultivars. Germination of conidia was maximized at 28°C on both cultivars by 48 hr with ZS 206 showing 100% germination, in contrast to only 63% germination in SC 625. As the number of days (1-5) of desiccation increased following inoculation, germination decreased from 100 to 47% in ZS 206 and from 62 to 0% in SC 625, respectively. The observation that C. zeae-maydis is able to tolerate unfavourable conditions and resume germ tube growth when favourable conditions return was confirmed in interrupted dew period studies. There was no change in percentage germination after 48 hrs., when plants were subjected to interrupted dew periods of 2-36 hrs, following a 6 hr period at 95-100% RH at 28 °C in a dew chamber. However, germination was lower (64%) on SC 625 than ZS 206 (90%). The wider range of temperature conditions favourable for conidial germination of ZS 206, and the fact that it was less affected by desiccation and interrupted dew periods than SC 625, could account for the different susceptibility levels of these two hybrids to GLS. Peak daily conidial catches were found to be between 1200 and 1400 hrs when temperatures and vapour pressure deficits were highest and leaf wetness lowest. Multiple regression analyses identified high evaporation over a 24 hr period, low temperatures over a 48 hr period and wind over a 72 hr period as the weather variables most strongly associated with high conidial releases. Rain, high vapour pressure deficit values and temperatures between 20-30 °C with leaf wetness over a 72-day period, together with prolonged high evaporation over a 48 hr period were identified as limiting factors in conidial release. These results indicate that temperatures (< 20 °C) and moisture 24-48 hrs prior to release is required for production of conidia. However, dry air and leaf surfaces are required for conidia to break off conidiophores at the point of attachment, i.e., a hygroscopic process is involved in release of conidia in C. zeae-maydis. In general, the process of conidiogenesis in C. zeae-maydis is similar to that observed on C. beticola. Successive formation of conidia on the same conidiophore are in accord with previous observations on C. zeae-maydis. Conidial measurements are also similar to other taxonomic descriptions of C. zeae-maydis. Hyphae aggregate in the substomatal cavity and give rise to fascicles of 1-2 septate conidiophore initials which emerge through the stoma. A single, aseptate conidium develops from the conidiogenous cell of the conidiophore initial. Extension growth of the conidiogenous cell from the base and one side of the terminal conidium, leads to the lateral displacement of the conidium on the conidiophore. After conidial secession, the conidiophore continues to grow, producing a second conidium from the conidiogenous cell at the apex of the extended conidiophore. This sympodial and successive proliferation of the fertile conidiogenous cell results in the formation of a characteristic 1-3 geniculate, occasionally 4, conidiophore, bearing a single conidium at each apex. This body of research has added information that was previously missing in the lifecycle of C. zeae-maydis. However, this additional information has, in turn, led to other yet unanswered questions which need to be addressed in the future, particularly under southern African conditions. A thorough knowledge and understanding of the epidemiology of this pathogen can result in more effective control strategies with increased yields for both commercial and small-scale farmers in KZN.
Studies on Phakopsora pachyrhizi, the causal organism of soybean rust.
(2006) Nunkumar, Archana.; Caldwell, Patricia May.; Pretorius, Zacharias Andries.
Phakopsora pachyrhizi H. Syd and P. Syd, the causal organism of soybean rust (SBR) was first reported in Japan in 1902. In 1934 the pathogen was found in several other Asian countries and as far south as Australia. In India, SBR was first reported on soybeans in 1951. There have been several early reports of SBR in equatorial Africa but the first confirmed report of P. pachyrhizi on the African continent was in 1996 from Kenya, Rwanda and Uganda. Since then, the pathogen has spread south with reports from Zambia and Zimbabwe in 1998 and in Mozambique in 2000. In February 2001, P. pachyrhizi was first detected on soybeans near Vryheid, in Northern KwaZulu-Natal, South Africa (SA). As the season progressed, the disease was observed in other parts of the province, and epidemic levels were found in the Cedara, Greytown, Howick and Karkloof production regions. Soybean rust subsequently spread to Amsterdam and Ermelo in the Highveld region of SA. The disease reappeared in SA in March 2002. It is now established that the pathogen is a threat to soybean production in the country with yield losses in the region of 10-80%. A literature review on SBR investigating the taxonomy of the pathogen, its morphology, symptoms, host range, infection process, epidemiology, control options and the economic importance of P. pachyrhizi was complied to provide the necessary background information to conduct research under local conditions and to assist in interpretation of results of experiments. Epidemiological trials were conducted at the University of KwaZulu-Natal under controlled environmental conditions in a dew chamber and conviron. Development of P. pachyrhizi on the susceptible cultivar (LS5995) was quantified in combinations of seven temperatures (15,19,21,24,26,28 and 30°C) and five leaf wetness durations (LWD) (6,9,12,14 and 16hrs) at three relative humidities (RH) (75%, 85% and 95%). Studies indicate that optimum temperature for uredospore infection is 21-24°C with a LWD greater than 12hrs and RH 85-95%. The number of pustules as well as lesion size on the abaxial and adaxial leaf surface increased with increasing LWD at all the RH values tested. Infection did not occur on plants incubated at 15°C and 30°C at 85% or 95%RH whereas at 75%RH infection did not occur on plants incubated at 15°C, 19°C and 30°C regardless of LWD. Number of pustules per lesion produced at 75%, 85% and 95%RH was highest at 24°C and showed a gradual increase with increasing LWD. Lesion size on both leaf surfaces increased after 12hrs LWD at 24°C at 75% and 85%RH whereas at 95%RH lesion size increased after 14hrs LWD at 24°C. Exposure of uredospores to ultraviolet light which is equivalent to ultraviolet C (sunlight) which is < 280nm, shows a decrease in germination (7%). Under continuous darkness, the germination percentage was found to range from 58% after 48 hrs. Germination was found to peak at 16hrs in darkness with a gradual decrease as time increased whereas germination under ultraviolet light was highest after 6hrs with a gradual decrease with increased exposure to light. Germ tube lengths were found to be shorter when exposed to ultraviolet light (107µm) compared to controls kept in the dark (181µm). Results obtained clearly show a negative effect of ultraviolet light on the germination and germ tube length of uredospores. A 0.1 ml suspension of uredospores on 1.25% water agar Petri dishes was exposed to cycles of 14h ultraviolet light and 10h darkness for 48h. Results indicate an increase in germination percentage of uredospores when exposed to 10h of darkness following a 14h period under ultraviolet light. Controlled environmental studies were conducted to determine alternative hosts of P. pachyrhizi in SA. The control used in this experiment was Prima 2000, a susceptible cultivar to soybean rust. Seven legume plants [Cajanus cajan (L.) Huth, Glycine max (L.) Merr, Lablab purpureus (L.) Sweet, Lupinus angustifolius (L.) Finnish, Phaseolus vulgaris (L.), Pueraria lobata (M&S) Wild and Vigna unguiculata (L.) Walp] and three dry bean lines (Bonus; OPS-RS2 and PAN 159) showed typical SBR symptoms when rated after 21 days post inoculation with uredospores for percentage disease severity. Disease severity was significantly different within the alternative hosts, but G. max, P. vulgaris and P. lobata were not significantly different from Prima 2000 (control). A uredospore suspension of 2.5 x 10(5) uredospores ml(-1) from plants that showed typical SBR symptoms was made and inoculated on to Prima 2000, a susceptible soybean cultivar. Uredospores from pustules on G. max, L. purpureus, L. angustifolius, P. vulgaris, P. lobata, V. unguiculata, Bonus and PAN 159 produced viable uredospores on PRIMA 2000. These plants are considered alternative hosts of P. pachyrhizi. Effect of leaf age on susceptibility of soybean to SBR was tested under controlled environmental conditions. Mean number of lesions as well as lesion size were greater on younger leaves than on older leaves of plants at the same physiological age. Plants at the early vegetative and reproductive stages had a significantly lower number of lesions as well as a smaller lesion size. Plants at the V6 and R1 growth stages were significantly more susceptible to P. pachyrhizi than plants at other developmental stages. Trichoderma harzianum Rifai, Eco-77® a commercial biological control product, was evaluated for its efficacy as a biological control agent of P. pachyrhizi. Trichoderma harzianum sprayed at the standard concentration on infected soybean plants was significantly more effective in controlling P. pachyrhizi than plants sprayed at 1/2X and 2x the standard concentration. This was noted in both Trial 1 and 2. Data indicate that spraying the filtrate two days after inoculation produces less disease.
Studies on Sclerotinia sclerotiorum (Sclerotinia stem rot) on soybeans.
(2007) Visser, Dael Desiree.; Caldwell, Patricia May.
Soybeans, Glycine max, are an economically and strategically important crop in South Africa (SA). In order to meet local demands, large imports of soybeans are required, e.g., in the 2005/2006 soybean production period, 842 107 tonnes of oilcake were imported. Due to an increase in soybean production throughout the world, diseases that affect this crop have also increased in incidence and severity. Sclerotinia sclerotiorum, the causal organism of sclerotinia stem rot (SSR), is an important yield limiting disease of soybeans, as well as numerous other crops. The pathogen was first reported in SA in 1979. However, it was only in 2002 that this fungus was considered a major pathogen of soybeans in SA. The research reported in this thesis was conducted to investigate the epidemiology of S. sclerotiorum and examine numerous potential control methods for this pathogen, i.e., resistant cultivars, biocontrol, chemical control and seed treatments. A S. sclerotiorum isolate was obtained from sunflowers in Delmas, Mpumulanga, SA, in the form of sclerotia. This isolate was cultured and sent for identification and deposition in the Plant Protection Research Institute collection. This isolate, in the form of mycelia, was used for the duration of the study. For epidemiology studies, the effect of temperature, leaf wetness duration (LWD) and relative humidity (RH) were examined for their effect on rate of pathogen development. Twenty four combinations of temperature (19°C, 22°C, 25°C and 28°C), LWD (24, 48 and 72 hr) and RH (85 and 95%) were investigated. No interaction between temperature, LWD and RH was found. Temperature alone was the only factor that affected disease development. At 22°C, the rate of pathogen development (0.45 per unit per day) was significantly higher than all other temperatures, indicating that this temperature is optimum for disease development. Thirteen different soybean cultivars, i.e., LS6626RR, LS6710RR, LS666RR, LS555RR, LS6514RR, LS678RR, Prima 2000, Pan 626, AG5601RR, AG5409RR, 95B33, 95B53 and 96B01B, commercially grown in SA were investigated for their reaction to S. sclerotiorum. Prima 2000, 96B01B, 95B33 and AG5409RR were considered to be the least susceptible as they showed a significantly low rate of pathogen development (0.28, 0.28, 0.24, 0.23 per unit per day, respectively) and produced a significantly low number of sclerotia (3.03, 3.42, 3.21, 2.38, respectively). LS6626R and LS666RR may be considered most susceptible because of their significantly high rate of pathogen development (0.45 and 0.42 per unit per day, respectively) and high sclerotia production (8.16 and 7.50, respectively). Regression analysis showed a positive correlation coefficient (R2=0.71) between rate of growth of the pathogen and number of sclerotia produced, indicating that a higher rate is associated with a higher number of sclerotia. In vitro dual culture bioassays were performed to identify the biocontrol mechanisms of the biocontrol agents, EcoT® (a seed treatment) and Eco77® (a foliar treatment), against hyphae and sclerotia of S. sclerotiorum. Ultrastructural studies revealed that mycoparasitism is the probable mode of action as initial signs of hyphae of EcoT® and Eco77® coiling around hyphae of S. sclerotiorum were observed. Surface colonization of sclerotia by hyphae of EcoT® and Eco77® was also observed. In vitro antagonism of EcoT® against S. sclerotiorum on soybean seed was performed to determine pre-emergence and post-emergence disease. There was no significant difference in percentage germination between seeds treated with EcoT® and plated with the pathogen, untreated seeds and no S. sclerotiorum, and the control (i.e. no EcoT® and no pathogen). However, percentage non infected seedlings from seeds not treated with EcoT® was significantly lower, suggesting that EcoT® may be successfully used as a seed treatment for the control of SSR. In vivo trials were performed to investigate the effect of silicon (Si) alone, and in combination with Eco77®, on the effect of the rate of disease development. Plants treated with Eco77® had a significantly lower rate of disease development (0.19 per unit per day for plants treated with Eco77® and S. sclerotiorum and 0.20 per unit per day for plants treated with Eco77®, S. sclerotiorum and Si), compared to plants not treated with Eco77® (0.29 per unit per day for plants treated with S. sclerotiorum and 0.30 per unit per day for plants treated with S. sclerotiorum and Si), regardless of the application of Si. Similarly, plants treated with Eco77® had a significantly lower number of sclerotia (0.46 for plants treated with Eco77® and S. sclerotiorum and 0.91 for plants treated with Eco77®, S. sclerotiorum and Si), compared to plants not treated with Eco77® (3.31 for plants treated with S. sclerotiorum and 3.64 for plants treated with S. sclerotiorum and Si). The significantly lower rate of disease development coupled with a significant reduction in sclerotia showed that Eco77®, and not Si, was responsible for reducing the severity of SSR. A strong positive correlation between rate of disease development and the number of sclerotia produced (R2=0.79) was observed. For the investigation of various fungicides for the control of S. sclerotiorum, in vitro trials to determine the potential of three different fungicides at different rates, i.e., BAS 516 04F (133 g a.i. ha-1), BAS 516 04F (266 g a.i. ha-1), BAS 512 06F (380 g a.i. ha-1) and Sumisclex (760 g a.i. ha-1) were initially conducted. The control (non-amended PDA) had a significantly higher area under mycelial growth curve (243.0) than all fungicides tested. BAS 516 04F (at both concentrations) and BAS 512 06F completely inhibited the mycelial growth of S. sclerotiorum. Sumisclex inhibited the fungus by 89.07%. For in vivo trials, preventative treatments, i.e., BAS 516 04F (133 g a.i. ha-1), BAS 516 04F (266 g a.i. ha-1), BAS 512 06F (380 g a.i. ha-1), curative treatment, i.e. Sumisclex (760 g a.i. ha-1) and a combination preventative/curative treatment, i.e., BAS 512 06F (380 g a.i. ha-1)/Sumisclex (570 g a.i. ha-1) were investigated. No significant difference in disease severity index (DSI) was found between fungicide treatments and the inoculated control. BAS 512 06F and BAS 512 06F/Sumisclex had significantly lower grain yields (6.09 g and 5.96 g, respectively) compared to all other treatments. There was a positive correlation coefficient (R2=0.76), between DSI and grain yield, indicating that a high DSI is correlated with low grain yield. Trials to evaluate the effect of commercially available and currently unregistered seed treatments for the control of S. sclerotiorum on soybean seeds in vivo and in vitro were performed. Seed germination tests were performed to determine if seed treatments had any negative effects on seed germination in vitro. All seed treatments tested, i.e., BAS 516 03F (8, 16 and 32 ml a.i. 100 kg-1 seed), BAS 512 00F (7.5, 15 and 32 ml a.i. 100 kg-1 seed), Celest XL (100, 125, 200 and 250 ml a.i. 100 kg-1 seed), Sumisclex (5 and 10 ml a.i. 100 kg-1 seed), Benomyl (150 g a.i. 100 kg-1 seed), Captan (240 ml a.i. 100 kg-1 seed), Thiulin (180 g a.i. 100 kg-1 seed) and Anchor Red (300 ml a.i. 100 kg-1 seed), showed no negative effect on seed germination. For in vivo trials, BAS 516 03F (16 and 32 ml a.i. 100 kg-1 seed), BAS 512 00F (7.5, 15 and 32 ml a.i. 100 kg-1 seed), Celest XL (100, 125, 200 and 250 ml a.i. 100 kg-1 seed), Sumisclex (5 and 10 ml a.i. 100 kg-1 seed), Benomyl and Anchor Red had significantly similar percent germination and percent seedling survival as the untreated/uninoculated control. These seed treatments should be recommended for the control of S. sclerotiorum, as they protected seed during germination and subsequent seedling development. BAS 516 03F (8 ml a.i. 100 kg-1 seed) should not be recommended for the control of SSR, as it gave the lowest percent germination and percent seedling survival. The results presented in this thesis have helped to identify optimal environmental conditions for the development of S. sclerotiorum, which is important for the development of forecasting models for disease control. The least and most susceptible cultivars of those tested have been identified. Biocontrol using Eco77® as a foliar application showed great potential. The effect of Si needs to be further investigated, including the testing of more frequent applications and higher concentrations. The fungicides tested in this research did not show any potential for the control of SSR. However, the spray programme tested is for the control of soybean rust (Phakopsora pachyrhizi), and was investigated for its potential for the control of SSR. The spray programme, fungicide application and rating scale needs to be modified, to determine the true potential of these fungicides for the control of SSR. Numerous seed treatments have shown potential for the control of seed infection by SSR. Due to difficulties in producing ascospores, which are the primary source of inoculum for this pathogen in the field, all studies in this research were conducted with mycelia and not ascospores. The production, collection and storage of ascospores needs to be thoroughly investigated, and research conducted with ascospores.
Studies on the biocontrol of seedling diseases caused by Rhizoctonia solani and Pythium sp. on sorghum and tef.
(2003) Tesfagiorgis, Habtom Butsuamlak.; Laing, Mark Delmege.; Caldwell, Patricia May.
Rhizoctonia solani and Pythium spp. are aggressive soil-borne fungal pathogens responsible for seed rot and seedling damping-off of many crops. With increased environmental and public concern over the use of chemicals, biological control of these diseases has been attracting more attention. However, success with this strategy depends on the development of effective antagonists, which requires repeated in vitro and in vivo tests. Bacillus spp. were isolated from a soil sample obtained from a field where sorghum and tef had been grown for at least two years. Potential Bacillus isolates were screened for their ability to inhibit in vitro growth of R. solani and Pythium sp. Among 80 isolates tested, endospore forming Bacillus spp. H44 and H51 gave highest antifungal activity against the two test-pathogens in three consecutive tests. Results demonstrated that both H44 and H51 have potential as biocontrol agents against diseases caused by these two pathogenic fungi. The interaction between three isolates of Trichoderma (T. harzianum Eco-T, Trichoderma spp. SY3 and SY4) and Pythium sp. were investigated using in vitro bioassays together with environmental scanning electron microscopy (ESEM). Visual observation on the dual culture tests revealed that hyphal growth of Pythium was inhibited by these antagonists soon after contact between the two organisms within 3-4 days of incubation. The ESEM investigations showed that all three isolates of Trichoderma grew toward the pathogen, attached firmly, coiled around and penetrated the hyphae of the pathogen, leading to the collapse and disintegration of the host's cell wall. Degradation of the host cell wall was postulated as being due to the production of lytic enzymes. Based on these observations, antibiosis (only by Eco-T) and mycoparasitism (by all three isolates) were the mechanisms of action by which in vitro growth of Pythium sp. was suppressed by these Trichoderma isolates. The reduction of seedling diseases caused by R. solani and a pythium sp. were evaluated by applying the antagonists as seed coating and drenching antagonistic Bacillus spp. (B81, H44 and H51) and Trichoderma (T. harzianum Eco-T and Trichoderma spp. SY3 and SY4). On both crops, R. solani and Pythium sp. affected stand and growth of seedlings severely. With the exceptions of H51, applications all of isoltes to seeds reduced damping-off caused by R. solani in both crops. Application of Eco-T, H44 and SY3 to sorghum controlled R. solani and Pythium sp. effectively by yielding similar results to that of Previcur®. On tef, biological treatments with Eco-T and SY4 reduced seedling damping-off caused by R. solani and Pythium sp., respectively, by providing seedling results similar to the standard fungicides, Benlate® and Previcur®. Most other treatments gave substantial control of the two pathogens on tef. Overall, Bacillus sp. H44 and T harzianum Eco-T were the best biocontrol agents from their respective groups in reducing damping-off by the two pathogens. In all instances, effects of application method on performance of biocontrol agents and adhesive on emergence and growth of seedlings were not significant. A field trial was conducted at Ukulinga Research Farm at the University of Natal, Pietermaritzburg, South Africa, to determine efficacy of biological and chemical treatments on growth promotion and reduction of damping-off incited by R. solani and Pythium sp., and to evaluate the effects of a seed coating material, carboxymethyl cellulose (CMC), on seedling emergence and disease incidence. Seeds of sorghum and tef were treated with suspensions of antagonistic Bacillus H44 or T harzianum Eco-T, or sprayed with fungicides, Benlate® or Previcur®. Application of Benlate® and Previcur® during planting significantly increased the final stand and growth of sorghum seedlings. Seed treatments with both H44 and Eco-T substantially controlled damping-off caused by Pythium, resulting in greater dry weights of seedlings than the standard fungicide. However, they had negative effects when they were tested for their growth stimulation and control of R. solani. The CMC had no significant effect on germination and disease levels. These results showed that these antagonists can be used as biocontrol agents against Pythium sp. However, repeated trials and better understanding of the interactions among the antagonists, the pathogens, the crop and their environment are needed to enhance control efficiency and growth promotion of these antagonists. Some of these biocontrol agents used in this study have the potential to diseases caused by R. solani and Pythium sp. However, a thorough understanding of the host, pathogen, the antagonist and the environment and the interactions among each other is needed for successful disease control using these antagonists.
The use of potato and maize disease prediction models using automatic weather stations to time fungicide applications in KwaZulu-Natal.
(2003) Van Rij, Neil Craig.; Caldwell, Patricia May.; Savage, Michael John.; Quinn, Nevil Wyndham.; Laing, Mark Delmege.
Maize grey leaf spot (GLS), caused by Cercospora zeae-maydis, and potato late blight (LB), caused by Phytophthora infestans, are foliar diseases of maize and potato, two of the most widely grown crops in KwaZulu-Natal (KZN), after sugarcane and timber. Commercial maize in KZN accounts for just on 4.3% of the national maize crop. This is worth R563 million using an average of the yellow and white maize price for the 2001/02 season (at R1 332.87 ton(-1)). In 2003 KZN produced about 5% of the national potato crop (summer crop: 7531 300 10kg pockets from 2243 hectares). This equates to a gross value of R89.4 million based on an average price of R1 188 ton(-1) in 2001. Successful commercial production of maize and potatoes depends upon control of these diseases by translaminar fungicides with highly specific modes of action. This study extends an existing model available for timing of fungicide sprays for GLS and tests and compares two LB models for two calendar-based spray programmes. The study also evaluated the use of an early blight model which is caused by Alternaria solani, and over the single season of evaluation showed potential for use in KZN. For the GLS model it was found that a number of refinements are needed, e.g., the amount of infected maize stubble at planting and not the total amount of maize residue at planting. Based on two years' data, it was found that for the LB models there are no significant differences in levels of control between using a predicted fungicide programme and a calendar-based programme. The importance of knowing initial infection sites, and hence initial inoculum, was demonstrated. This led to the creation of a KZN LB incidence map, now being used to more accurately time the start of a preventative spray programme and to time the inclusion of systemic fungicides in the preventative spray programme. This study has contributed to the further development and expansion of the Automatic Weather Station Network (AWSN) at Cedara, which now comprises 15 automatic weather stations in KZN. The AWSN is currently used to aid farmers and advisers in decision-making regarding fungicide spray timing for GLS and LB.